The reaction between diphosphorus derivatives [(ClImDipp)P2(Dipp)]OTf (1[OTf]) and [(ClImDipp)P2(Dipp)Cl] (1[Cl]) with the cyclotetraphosphido cobalt complex [K(18c-6)][(PHDI)Co(& eta;4-cyclo-P4)] (2) leads to the formation of complex [(PHDI)Co{& eta;4-cyclo-P6(Dipp)(ClImDipp)}] (3), which features an unusual hexaphosphido ligand [ClImDipp=4,5-dichloro-1,3-bis(2,6-diisopropylphenyl)imidazol-2-yl, Dipp=2,6-diisopropylphenyl, 18c-6=18-crown-6, PHDI=bis(2,6-diisopropylphenyl)phenanthrene-9,10-diimine]. Complex 3 was obtained as a crystalline material with a moderate yield at low temperature. Upon exposure to ambient temperature, compound 3 slowly transforms into two other compounds, [K(18c-6)][(PHDI)Co(& eta;4-P7Dipp)] (4) and [(PHDI)Co{cyclo-P5(ClImDipp)}] (5). The novel complexes 3-5 were characterized using multinuclear NMR spectroscopy and single-crystal X-ray diffraction. To shed light on the formation of these compounds, a proposed mechanism based on 31P NMR monitoring studies is presented. [4+2] condensation reaction of diphosphene [(ClImDipp)P2(Dipp)]OTf and diphosphanide [(ClImDipp)P2(Dipp)Cl] with the cyclo-P4 cobalt complex [K(18c-6)][(PHDI)Co(& eta;4-cyclo-P4)] forms a CoP6 complex, featuring an unusual hexaphosphido ligand composed of a cyclo-P5 ring and an exocyclic (ClImDipp)P moiety. 31P NMR spectroscopic studies reveal that at ambient temperature, this CoP6 complex undergoes disproportionation into more stable CoP7 and CoP5 complexes.+image